You could try extending the work area, but why not just put a black matte and do the job properly?
If you play out the timeline to tape, a very short black clip at the end will generate permanent black output to tape, so you can avoid longer timelines that way if you wish. But exporting to file needs all frames to be specified.
A Title would work, as would Black Video, or White Video. As Jim asks, why would you NOT want to use Black Video and do Dip-to-Black to get there? One thing that you do not want is a gap on the Timeline in your Video Track. If you Audio is longer, you do want something and Black Video (or a variation) is good.